(a) Field of the Invention
The present invention generally relates to an amplifier, and more particularly, to a method for canceling third order nonlinearity components in an amplifier.
(b) Description of the Related Art
One of the important matters that should be considered in designing analog integrated circuits using semiconductors is to output incoming signals without distorting them, which is called linearity of a circuit. Though an ideal amplifier satisfies a linear function relationship between an output signal y and an input signal x as represented by equation (1), Tayler series are developed as equation (2) in an actual circuit.y=gx  (1)y=g1x+g2x2+g3x3  (2)where terms of more than second order, g2x2 and g3x3, represent nonlinear distortion in the actual circuit.
FIG. 1A shows a conventional common source amplifier that is composed of an amplifier transistor M1 and a load. FIG. 1B is a graph showing the relationship between input voltage Vin applied to the gate of the transistor M1 of FIG. 1A and output current Iout. In FIG. 1B, curves G1, G2, and G3 represent factors g1, g2, and g3 of equation (2) in DC, respectively. The available biasing range of an amplifier is determined from an appropriate relationship between consumption power and amplification gain. That is, high output current Iout increases consumption power in the range where the input voltage Vin is too high, and small first order linearity component G1 decreases gain amplification in the range where the input voltage Vin is too low. Thus, the available bias is determined at the point where the input voltage Vin is about 0.7V (±0.2V) according to trade-off of the output current Iout and first order linearity component G1 as shown in FIG. 1B.
Where the input voltage Vin is about 0.7V as shown in FIG. 1B, however, third order nonlinearity component G3 has a maximum magnitude in the active region, and second order nonlinearity component G2 also has relatively large magnitude. The second order nonlinearity hardly generates in general analog integrated circuits, since most of them operate based on differential signal processing. In addition, the second order nonlinearity is an insignificant problem in modulation and demodulation in the current radio communication system. However, third order nonlinearity is a significant matter because it generates signal distortion in a transmission/reception band due to intermodulation of neighboring channel signals.
A conventional technique for canceling the third order nonlinearity to improve linearity of a circuit is to increase DC current. However, this technique is not suitable for portable devices that require low power consumption because a high DC current increases power consumption. To solve this problem, a method for canceling nonlinearity using an additional circuit without supplying high DC current has been proposed. However, a general additional circuit has a complicated structure so that the overall area of the amplifier becomes larger, or consumption power increases. Accordingly, a method employing a simple additional circuit is required.